This invention relates to optical signal mode converters with very high efficiency tuning capability.
Tunable broadband mode-converters play an important role in WDM optical communication systems. They may be used to dynamically convert a lightwave signal propagating in one mode of a few-mode fiber, into another spatial mode. Such coupling is attractive to alter the path the lightwave signal takes, because the alternate path (defined by another spatial mode in the fiber) may have preferred dispersion, nonlinearity, or amplification properties. An example of this is a higher-order-mode dispersion compensator, where light in an entire communications band is switched from an incoming LP01 mode to a higher-order-mode (HOM) such as the LP11 or LP02 mode. See for example: C. D. Poole et al., J. Lightwave Tech., vol 12, pp.1746-1758 (1994); S. Ramachandran, et al., IEEE Photon. Tech. Lett., vol 13, pp. 632-634 (2001); A. H. Gnauck et al., Proc. Opt. Fiber Comm., PD-8 (2000); U.S. Pat. Nos. 5,185,827, 5,802,234.
In a general sense, long-period gratings are mode-conversion devices that provide phase-matched coupling to transfer power from one mode of an optical fiber to another. (See, e.g., J. N. Blake, B. Y. Kim and H. Shaw, xe2x80x9cFiber-Optic Modal Coupler Using Periodic Gratings,xe2x80x9d Opt. Lett. 11,177(1986); J. N. Blake, B. Y. Kim, H. E. Egan, and H. J. Shaw, xe2x80x9cAll-Fiber Acusto-Optic Frequency Shifter,xe2x80x9d Opt. Lett. 11, 389(1986); and J. N. Blak, B. Y. Kim, H. E. Egan, and H. J. Shaw, xe2x80x9cAnalysis of Intermodal Coupling in a Two-Mode Fiber with Periodic Microbends,xe2x80x9d Opt. Lett. 12, 281(1987)). This has proven to be especially useful for coupling between a guided mode and a cladding mode of ordinary transmission fibers, to create wavelength selective loss (See, e.g., M. Tachibana, R. I. Laming, P. R. Morkel and D. N. Payne, xe2x80x9cErbium-Doped Fiber Amplifier with Flattened Gain Spectrum,xe2x80x9d IEEE Phot. Tech. Lett. 3, 118(1991)). In optical communications systems, LPGs have been used extensively for realizing devices that offer wavelength-selective attenuation of a WDM communications signal.
Most of the applications for LPGs have concentrated on static wavelength attenuation. Dynamic tuning of the spectral characteristics of LPGs has been proposed, and a variety of dynamic tuning techniques have been demonstrated. LPGs that couple the core mode to a cladding mode can be tuned dynamically by modulating the refractive index of an outer or inner cladding material that is interrogated by a cladding mode of the fiber. The refractive index of such cladding materials can be varied by temperature, the electro-optic effect or some nonlinear optical effect, depending on the nature of the cladding material used. Alternately, the LPGs may be strained by piezoelectric packages, simple motion control housings or magnetically latchable materials, to tune the core-to-cladding resonance. All these tuning techniques have been applied to LPGs coupling core modes to cladding modes, and offer tunable attenuation over a limited, narrow spectral range. The tuning mechanisms described above serve to shift the spectral response of LPGs from one wavelength to another. While these techniques are useful for tuning the wavelength selective attenuation in a fiber-optic system, they cannot be used for broadband mode-conversion schemes. This is because the devices transform light into a cladding mode, and cladding mode transmission is known to be inefficient. Thus these devices are not useful in systems that propagate signals over long lengths, as are required for devices such as the HOM dispersion compensators. In addition, the spectral width of mode coupling with current tunable LPGs is undesirably narrow. Typical bandwidths are xcx9c1 nm for 99% mode-conversion, while a practical device would need more than a 40 nm bandwidth. While chirped LPGs have been proposed to enhance the bandwidth, the approach introduces an inherent trade-off between bandwidth and strength of mode-conversion. Most importantly, the tuning that is most desirable for dynamic filters is in the strength of the coupling, and not the resonant wavelength. The devices described above provide only the latter form of tunability.
Broadening the bandwidth of LPGs by coupling to a higher-order cladding mode has been described by V. Grubsky et al., xe2x80x9cLong period fiber gratings with variable coupling for real-time sensing applications, Optics Lett., Vol. 25, p. 203 (2000). In this device, greater than 50-nm coupling has been achieved, albeit with weak coupling strengths. The coupling strength was tuned by temperature or strain, but the device suffered from the drawback that it coupled to a cladding mode, which is lossy in nature. The spectral characteristics of this device were controlled by the silica cladding of a fiber. This structure is not amenable to arbitrary control, and thus the spectral shape or characteristics could not be altered, as would be required of a practical mode-converter.
Thus, there exists the need for a fiber-grating device that can offer strong broadband coupling, preferably over bandwidths exceeding 30 nm, whose coupling strength is tuned by temperature, strain, the electro-optic effect, the nonlinear optic effect, or any other means that modifies the refractive index of a material. A practical device would offer mode-conversion such that the converted mode can be propagated for long distances without significant attenuation.
According to the invention, a few mode fiber is used for the mode converter, and coupling is made between a fundamental, or near fundamental, propagation mode and the next, or closely adjacent, higher order mode (HOM). Both modes propagate in the core of the optical fiber, thus maintaining efficient transmission. Mode coupling is effected using a long period grating (LPG) and the strength of the mode coupling is dynamically varied by changing the period of the grating or by varying the propagation constants of the two modes being coupled. The period of the grating is varied by physically changing the spacing between grating elements, for example by changing the strain on the grating to physically stretch the LPG. The propagation constants of the modes can be varied using any method that changes the refractive index of the fiber containing the LPG, for example, by changing the temperature, electrically changing the index using the electro-optic effect, or optically changing the index using the non-linear optic effect. In every case the two modes being coupled are core modes with high propagation efficiency.
In the following description an LPG formed in a few mode fiber is referred to as an HOM-LPG.